1.Field of the Invention
The present invention relates to apparatus and method for manufacturing a hermetically coated optical fiber by coating a thin film of carbon or the like on an optical fiber spun from an optical fiber preform.
2. Related Background Art
As a technique to hermetically coat carbon to a spun bare fiber, a method for coating a carbon thin film on a surface of a bare fiber by exposing the optical fiber to a high temperature carbon gas atmosphere in a process of thinning the optical fiber has been known. (For example, see JP-B-38-10363)
Ep 0.308.143 (published on Mar. 22, 1989) discloses a method for manufacturing a hermetically coated optical fiber by guiding a thinned high temperature bare fiber into a carbon gas atmosphere in a process of spinning the optical fiber from an optical fiber preform to coat a carbon thin film on a surface of the bare fiber. In one embodiment of the disclosed method, a reaction tube having two reaction chambers is used, raw gas is separately introduced into each of the reaction chambers and the carbon thin films are coated in two steps in the respective reaction chambers.
In the past, when a hermetic coating is applied by carbon on the surface of the optical fiber, a residual stress appears in an interface of the carbon film and the optical fiber or a tensile strength of the fiber is reduced by the unevenness created on the surface of the carbon film.
In the related art, since a reaction condition such as concentration and composition of raw gas in one reaction chamber is kept constant the carbon film formed on the bare fiber has a uniform property.
Where two or more reaction chambers are provided and carbon is coated with different gas atmospheres for the respective reaction chambers as proposed by the related art, heterogeneous films with interfaces are formed because the fiber temperature varies from chamber to chamber. As a result, a distortion due to a residual stress is created at a heterogeneous phase interface and the improvement of strength is not attained.